A typical hard disk drive includes a head disk assembly (“HDA”) and a printed circuit board assembly (“PCBA”). The HDA includes at least one magnetic disk (“disk”), a spindle motor for rotating the disk, and a head stack assembly (“HSA”) that includes a head having a slider and at least one transducer or read/write element for reading and writing data. The HSA is controllably positioned by a servo system in order to read or write information from or to particular tracks on the disk. The typical HSA has three primary portions: (1) an actuator assembly that moves in response to the servo control system; (2) a head gimbal assembly (“HGA”) that extends from the actuator assembly and biases the slider toward the disk; and (3) a flex cable assembly that provides an electrical interconnect with minimal constraint on movement.
A typical HGA includes a load beam, a flexure (also called a gimbal) attached to an end of the load beam, and a head attached to the flexure. The load beam has a spring function that provides a “gram load” biasing force and a hinge function that permits the air bearing surface of the slider to follow the surface contour of the spinning disk. The load beam has an actuator end that connects to the actuator arm and a flexure end that connects to the flexure that supports the head and transmits the gram load biasing force to the head to “load” the slider against the disk. Air near the spinning disk is drawn between the air bearing surface of the slider and the disk, developing regions of super-ambient pressure beneath the surface of the slider that push the slider slightly away from the disk in opposition to the gram load biasing force. The slider is said to be “flying” over the disk when in this state.
Early HGAs included a number of twisted wires within a tube attached to a side of the actuator arm to electrically couple the slider to the preamplifier. However, more recent developments in the disk drive industry, such as the continuing miniaturization of slider assemblies (including the head and the transducer) and the transition to magnetoresisitive (MR) heads that require more electrical connections, have led to abandoning such configurations in favor of a configuration in which conductive traces are laid on a polyimide film formed on or coupled to the flexure. Such technologies are variously named TSA (Trace Suspension Assembly), CIS (Circuit Integrated Suspension), FOS (Flex on Suspension) and the like. Whatever their differences, each of these technologies replaces the discrete twisted wires with conductive traces (copper, for example) and insulating material (such as polyimide, for example) and support or cover layers (including stainless steel, for example). These conductive traces interconnect the transducer elements of the head to the drive preamplifier and the circuits associated therewith.
The HSA is internal to the HDA in a very clean environment within the drive. The PCBA, however, is outside the HDA—and outside of the clean environment. One problem faced by hard disk drive designers is how best to electrically connect the HSA to the PCBA without compromising the clean environment of the HDA, and to do so in a reliable, readily manufacturable and inexpensive manner. The head stack in a disk drive rotates around a pivot axis, and a flat flexible cable is required to allow for the pivot motion while maintaining the electrical connection. This requirement places the major surfaces of the flat flexible cable in a vertical orientation.
That portion of the flexure that attaches to the flex cable is commonly called the tail. There are three commonly used approaches to attach the flexure tail to the flexible cable. These are ultrasonic wire bond, solder reflow, and anisotropic conduct film. Of these, the ultrasonic wire bond and solder reflow approaches are most popular. An important difference is that pressure is applied to the joint when joining the suspension tail to the flex cable using the ultrasonic wire bond technique, whereas only heat is applied to the joint using the solder reflow technique. Due to suspension design miniaturization and the increase in the number of conductor leads, the tail termination pads on which the solder bumps are disposed have and should continue to become much smaller in size. To maintain equivalent joint strength and keep manufacturing robustness, non-contact solder reflow is getting more attention and is implemented by more head stack assembly suppliers.
Solder reflow can be achieved either in plane (such as that in flip-chip) or in 90 degree (solder fillet). For the solder reflow in the 90 degree configuration, the contact pressure and compliance of the tail portion against the flexible cable will create a preload which will force the solder bump on flexure tail against the flexible cable pad or solder bump. This preloading force helps to insure a good solder joint after solder reflow. However, edge irregularities or stacking tolerances might cause some pads to contact while preventing other pads on the tail portion of the flexure from making good contact with the corresponding pad or solder bump on the flex cable.
FIG. 1A is a plan view of elements of a conventional head stack assembly. As shown therein, the head stack assembly includes an actuator arm 102, to which a load beam 103 is attached. A flexure 106 including a plurality of conductive traces is at least partially supported by the load beam 103. FIG. 1A also shows the tail portion 108 of the flexure 106. The flexure may have a laminar construction, in which the conductive traces are disposed on an insulating layer that is, in turn, disposed on a support layer. FIG. 1B shows the outline of the support layer 110 of the tail portion 108 of the flexure 106 of FIG. 1A. As may be seen in FIG. 1B, the tail supports the connection pads (shown in outline form in FIG. 1B), which are configured to connect to corresponding pads on the flexible cable, to connect the head stack assembly to the PCBA. As seen in FIG. 1C, the connection pads 118 are disposed on an insulating layer 114, which is layered on the support layer 110. The support layer 110 of the tail 108 defines a plurality of small projections 111. However, the connection pads 118 and the solder bumps 120 disposed thereon are disposed on a single continuous and homogeneous slab of support material. Should the aforementioned edge irregularities and stacking tolerances be too great, one or more of the solder bumps 120 may be unable to make a good contact with corresponding solder bumps on the mating portion of the flexible cable. As the number of contacts within the suspension tail-flexible cable interface increases, it becomes increasingly difficult to control the height of the solder bumps 120 to insure uniformity. Ultrasonic tab bonding techniques become more difficult to carry out as well, as the number of wires increase and the spacing therebetween correspondingly decreases.
FIG. 2A shows elements of a conventional flexure tail such as shown, for example, in published patent application US 2004/0257708 A1. As shown in FIG. 2A, the support layer 202 defines a bounded and enclosed cutout region 203 into which project a plurality of fingers 204. FIG. 2B shows an imaginary plane 208 that is oriented perpendicularly relative to the support layer 202 and disposed between adjacent ones of the fingers 204. As shown, the plane 208 extends beyond the free end of the tail. In so doing, the plane 208 cuts through the support layer material 202, both through the support layer material at the free end of the tail portion and through the support layer 202 that extends between adjacent fingers 204. Such a construction, however, may be better suited to plated solder, and may not be capable of screen solder application, due to small footprint of the support material 202 defining the fingers 204. Moreover, although the fingers 204 operate as individual springs or gimbals, the supporting leading portion 206 of the tail will still contact to the flex cable as a whole, which may limit the compliance of the individual fingers 204.
From the foregoing, it may be appreciated that improved disk HSAs, HGAs, suspensions and disk drives are needed that are well suited to solder reflow (i.e., non-contact) bonding techniques and that are readily scalable to accommodate a greater number of smaller conductive traces. Such improved suspensions and suspension containing device should not require expensive retooling and should allow a preload of the solder bumps or connection pads against the corresponding structures on the flexible cable, to insure a good electrical contact between the two structures prior to the application of heat to cause the solder reflow. Moreover, in view of the decreasing size and separation between adjacent tail connection pads in new and forthcoming drive designs, such improved suspensions should allow ready use of solder reflow and/or other non-contact bonding techniques.